Exercise has been increasingly promoted as an important component of a weight reduction program in addition to dieting. However, little is known to help resolve the controversy about the mode of exercise to recommend. We plan to compare strength training with isoenergetic aerobic exercise during restrictive dieting for 8 weeks in 60 men and 60 women (ages 18-400. These subjects will be moderately obese and otherwise in good health. They will receive a liquid formula diet that provides either 50% or 70% of resting metabolic rate (RMR) corresponding to a mean of 900 or 1250 kcal/day. Dieting alone, especially at low caloric levels, leads to a loss of lean body mass (LBM), as well as fat. Since RMR is related largely to the amount of LBM, strength training, which stimulates muscle growth, may help diminish the reduction of LBM and RMR, as compared to aerobic exercise. Alternatively, aerobic exercise may enhance cardiorespiratory capacity and lower fasting insulin and glucose, cholesterol and cholesterol/HDL ratio more than strength training. Subjects will be randomized to do strength training, aerobics or no exercise. A group of control subjects will eat ad libitum and not exercise. All measurements will be made prior to and at the end of the study. Overnight RMR will be determined in our metabolic chamber followed by a standard RMR. Then body composition will be assessed by underwater weighing, total body water, total body potassium, dual X-ray absorptiometry, and anthropometric indices. Estimates will made of upper arm muscle circumference and total muscle mass as well as determinations of grip, arm, and leg strength. Measurements of maximal oxygen consumption will also be performed. Psychological states will be assessed including depression and other moods, body image, subject compliance, and degree of hunger during the study. Follow-up at 12 months will determine the persistence of the changes in weight, body composition, and RMR as well as dietary and exercise habits. The results should provide evidence for the appropriate training mode and caloric level in subjects undergoing weight reduction.